Fonts, like all forms of creative expression, are covered by copyright unless their copyright owner explicitly releases them into the public domain. Distributing a font, or derivative works of the font (bitmap images of text rendered using the font, for example) would be a violation of copyright, unless you have a license to do so.
The particular font you mention, Trajan Pro, was created by Carol Twomby. A license for this font (and several of her other fonts) can be purchased from Adobe.
According to the licensing terms, this font license gives permission to embed the font into an electronic document for the purposes of preview and print, but not for the font to be installable by any end-users of such a document. It does not explicitly grant the right to (for example) be baked into a texture page and used in a game, so it's not obvious whether even purchasing a license would allow you to legally use this font the way that you wish to. As usual for this sort of thing: consult a practising lawyer in your area for an authoritative answer of whether a game constitutes an "electronic document" for the purposes of this font license.
Similar discussions have been held on the official Adobe discussion forums a few years back, but never received an official reply from Adobe.